Basic Principles of Batteries and Electrochemical Cells
Batteries may be divided into two principal types, primary batteries and secondary batteries. Primary batteries may be used once and are then exhausted. Secondary batteries are also often called rechargeable batteries because after use they may be connected to an electricity supply, such as a wall socket, and recharged and used again. In secondary batteries, each charge/discharge process is called a cycle. Secondary batteries eventually reach an end of their usable life, but typically only after many charge/discharge cycles.
Secondary batteries are made up of an electrochemical cell and optionally other materials, such as a casing to protect the cell and wires or other connectors to allow the battery to interface with the outside world. An electrochemical cell includes two electrodes, the positive electrode or cathode and the negative electrode or anode, an insulator separating the electrodes so the battery does not short out, and an electrolyte that chemically connects the electrodes.
In operation the secondary battery exchanges chemical energy and electrical energy. During discharge of the battery, electrons, which have a negative charge, leave the anode and travel through outside electrical conductors, such as wires in a cell phone or computer, to the cathode. In the process of traveling through these outside electrical conductors, the electrons generate an electrical current, which provides electrical energy.
At the same time, in order to keep the electrical charge of the anode and cathode neutral, an ion having a positive charge leaves the anode and enters the electrolyte and a positive ion also leaves the electrolyte and enters the cathode. In order for this ion movement to work, typically the same type of ion leaves the anode and joins the cathode. Additionally, the electrolyte typically also contains this same type of ion. In order to recharge the battery, the same process happens in reverse. By supplying energy to the cell, electrons are induced to leave the cathode and join the anode. At the same time a positive ion, such as Na+, leaves the cathode and enters the electrolyte and a Na+ leaves the electrolyte and joins the anode to keep the overall electrode charge neutral.
In addition to containing an active material that exchanges electrons and ions, anodes and cathodes often contain other materials, such as a metal backing to which a slurry is applied and dried. The slurry often contains the active material as well as a binder to help it adhere to the backing and conductive materials, such as carbon particles. Once the slurry dries it forms a coating on the metal backing.
Unless additional materials are specified, batteries as described herein include systems that are merely electrochemical cells as well as more complex systems.
Anodes in Sodium-Ion Batteries
In order for a battery to function properly, the materials used in the anode, cathode and electrolyte are typically selected to have compatible electrical, chemical, and electrochemical properties. For instance, the materials may be selected to operate at compatible voltages. A variety of anodes have been developed for lithium-ion batteries and many varieties of such batteries have been commercially successful. The same is not true, however, of sodium-ion batteries, which often require a different anode or cathode than lithium-ion batteries due to a variety of differences between the lithium ion (Li+) and the sodium ion (Na+). In particular, the larger radius of Na+ makes it difficult to find an electrode material that can reversibly contain Na+. This had resulted in the development of very few commercially available sodium-ion batteries.
Examples of commercially available sodium-ion batteries include the Na/S battery and the Na/NiCl2 battery (also called the ZEBRA battery). These batteries require the use of molten materials that are difficult to maintain. The use of simpler materials, such as carbon-based anodes, has been investigated, but has not resulted in a commercially viable product. For instance, a great deal of research has focused on hard carbon anodes (e.g. non-graphitizable carbon), which can deliver a reversible capacity of 200-300 mAh/g, but which suffer from poor reversibility. Similarly, graphite has proven to be unsuitable anode material because the large sodium ions cannot enter and intercalate properly. Due to the voltages at which sodium-ion batteries must operate, which is close to the voltage at which sodium ions convert to metallic sodium, all sodium ion batteries with a carbon anode also suffer from sodium plating, resulting in the formation of a solid-electrolyte interfacial (SEI) layer, which impede sodium ion movement, or dendrites, which may cause dangerous short circuits in the battery. Carbon anodes suffer from the further drawback of manganese poisoning when used with manganese-containing cathodes, which limits battery life.
Recently, in lithium-ion batteries, a new type of anode in which metallic and intermetallic materials that react with Li+ to form an alloy have showed promise, but like other anodes, these materials must allow passage of the ion in order to function. Accordingly, this type of material has not previously been investigated for the much larger Na+, which, as discussed above, does not readily function with most lithium-ion battery anodes.